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Abstract:

γ-Hydroxycarboxylic esters and γ-lactones which are suitable
as flavors can be prepared by electrochemical reductive cross-coupling of
α,β-unsaturated esters with carbonyl compounds in an undivided
electrolysis cell having a cathode composed of lead, lead alloys,
cadmium, cadmium alloys, mercury, steel, glassy carbon or boron-doped
diamonds and a basic aqueous electrolyte comprising an electrolyte salt
which suppresses the cathodic formation of hydrogen.

Claims:

1. A γ-butyrolactone derivative of formula I ##STR00012## wherein
R1, R2 and R3 are each independently a hydrogen or an alkyl group having
from 1 to 5 carbon atoms,. and R4 and R5 are alkyl groups having from 1
to 4 carbon atoms, with R4 and R5 being identical radicals.

2. The γ-butyrolactone derivative according to claim 1, wherein the
derivative is selected from the group consisting of
4-(2-pentyl)butyrolactone and 3 -methyl-4-(2-pentyl)butyrolactone.

3. The γ-butyrolactone derivative according to claim 1, wherein the
derivative is a flavor.

4. A γ-hydroxycarboxylic acid or γ-hydroxycarboxylic ester of
formula VII ##STR00013## wherein R1, R2, R3 and R7 are each
independently, a hydrogen or an alkyl group having from 1 to 5 carbon
atoms, R is a hydrogen or an alkyl group, and R8 is a branched alkyl
group having from 3 to 10 carbon atoms.

5. The γ-hydroxycarboxylic acid or γ-hydroxycarboxylic ester
according to claim 4, wherein R7 is a hydrogen.

Description:

CROSS REFERENCE TO RELATED APPLICATION(S)

[0001] This application is a divisional of U.S. application Ser. No.
13/594,028, filed on Aug. 24, 2012, which incorporates by reference the
provisional U.S. application 61/526722 filed on Aug. 24, 2011, and claims
foreign priority to EPO 11178688.5 filed on Aug. 24, 2011, the entire
content of which is incorporated herein by reference.

DESCRIPTION

[0002] The invention relates to a process for the electrochemical
preparation of y-hydroxycarboxylic esters and γ-lactones by
reductive cross-coupling of α,β-unsaturated esters with
carbonyl compounds in an undivided electrolysis cell, in which a cathode
composed of lead, lead alloys, cadmium, cadmium alloys, mercury, steel,
glassy carbon or boron-doped diamonds and a basic aqueous electrolyte
comprising an electrolyte salt selected from among bisquaternary and
multiquaternary ammonium and phosphonium salts are used.

[0003] The invention further relates to the y-butyrolactone derivatives of
the formula I

##STR00001##

[0004] which can be prepared by the process of the invention, and also
their use as flavors.

[0005] The invention also relates to the γ-hydroxycarboxylic acids
or γ-hydroxycarboxylic esters of the formula VII

##STR00002##

[0006] which can likewise be prepared by the process of the invention.

[0007] The industrially most important γ-lactone is
γ-butyrolactone. It is prepared industrially either by
dehydrocyclization of 1,4-butanediol in the gas phase or by hydrogenation
of maleic anhydride. A further classical method for preparing
γ-lactones is the alkaline hydrolysis of γ-halocarboxylic
acids.

[0008] The above-described methods always go out from an existing
disubstituted C4 framework, so that substitution patterns on the ring
cannot be realized convergently. However, methods in which the future
lactone ring is built up only by means of C,C-bond coupling are also
known. These include, for example, the oxidative coupling of acetic acid
with olefins (C2+C2) or the tert-butyl hydroperoxide-aided cyclization of
acrylic acid with alcohols (C3+C1). In these cases, the substitution of
the ring can be controlled by clever use of the appropriate starting
materials in the cyclization.

[0009] This type of lactone synthesis also includes the reductive coupling
(dihydrodimerization) of acrylic esters and carbonyl compounds according
to the following reaction scheme:

##STR00003##

[0010] The reductive coupling of acrylic acid derivatives with carbonyl
compounds can be effected by means of reducing agents such as magnesium
or samarium(II) iodide. Electrochemical methods which avoid the
stoichiometric use of a chemical reducing agent have also been described.
Fundamental metal studies in this field were carried out in a divided
electrochemical cell at a mercury pool cathode in a sulfuric acid
electrolyte at cathodic current densities of up to 2.8 A/dm2. In the
studies, the reductive coupling of acrylonitrile with acetone was
observed, which thus do not yet lead to the lactones.

[0011] Proceeding herefrom, Shono et al. (Tetrahedron Lett. 1980, 21,
5029-5032) have described the reductive coupling of
α,β-unsaturated esters with aldehydes or ketones in a divided
electrochemical cell. The electrolyte used was based on
N,N-dimethylformamide (DMF) with N,N,N,N-tetraethylammonium
toluenesulfonate (Et4NOTs) as electrolyte salt. Furthermore,
stoichiometric amounts of a chlorosilane (trimethylsilyl chloride, TMSCI)
were added to activate the carbonyl component. The electrolysis was
carried out at a current density of 0.4 A/dm2, which is far removed
from industrially relevant current densities of >1 A/dm2. Nobuya
et al. (JP 57108274 A) have undertaken a further step toward industrial
implementation by using a water-based electrolyte. The preparation of the
lactones was carried out in a divided electrolysis cell at current
densities of 10 A/dm2. Here, an acidic anolyte (e.g. 10% strength
H2SO4) and a KH2PO4-buffered catholyte were used. In divided cells, the
two electrode spaces are separated by a membrane. Undivided cells are
cheaper and industrially easier to realize. Particularly in the case of
organic processes, rapid aging of the membrane and therefore
unsatisfactory operating lives can be expected.

[0012] U.S. Pat. No. 4,414,079 describes the reaction of
α,β-unsaturated esters with aldehydes in an undivided cell
using, for example, tetra-n-butylammonium sulfate as electrolyte salt. In
a further approach, Burger (Katrin Burger, Thesis 2003, Universitat
Munster) has carried out the reaction of α,β-unsaturated
esters with aldehydes or ketones in an undivided cell. Electrolytes used
were binary mixtures of alcohols (e.g. methanol or ethanol) with water or
dioxane and also high concentrations of electrolyte salts (e.g.
tetrabutylammonium tetrafluoroborate, Bu4NSF4). Interestingly,
graphite electrodes were used in the system described and, owing to their
comparatively high hydrogen overvoltage, these could serve as
alternatives for lead, mercury and cadmium electrodes. However, the
yields of lactone which can be obtained by this process are
unsatisfactory since the corresponding homo-coupling products and the
reduced carbonyl component (i.e. the corresponding alcohol) are formed to
a large extent as by-products, even though the homo-coupling of the
α,β-unsaturated esters is countered by a high excess of
carbonyl compound. In addition, this process is based on the use of
electrolytes based on binary organic solvents (alcohol with water or
alcohol with dioxane), which makes a complicated separation of the
product from the solvent necessary after the electrolysis. The use of
alcohol-comprising solvents is also disadvantageous because the alcohol
is oxidized (to aldehyde and further) in the electrolysis. As a result,
expensive solvent is lost and the aldehyde formed has to be separated off
in a complicated manner.

[0013] It is therefore an object of the invention to provide a process for
the electrochemical preparation of the γ-lactones and
γ-hydroxycarboxylic esters by cross-coupling of
α,β-unsaturated esters with carbonyl compounds, in which the
disadvantages of the prior art, in particular the use of divided
electrochemical cells, of low current densities (<1 A/dm2) and
the occurrence of yield-reducing secondary reactions are avoided. This
object is achieved by the claimed embodiments described below.

[0014] The present invention accordingly provides a process for the
electrochemical preparation of γ-hydroxycarboxylic esters and/or
γ-lactones by reductive cross-coupling of
α,β-unsaturated esters with carbonyl compounds in an undivided
electrolysis cell, wherein the cathode material is selected from the
group consisting of lead, lead alloys, cadmium, cadmium alloys, mercury,
steel, glassy carbon and boron-doped diamonds and a basic, aqueous
electrolyte comprising at least one electrolyte salt selected from among
bisquaternary and multiquaternary ammonium and phosphonium salts is used.

[0015] For the purposes of the present invention, a carbonyl compound is
an aldehyde or a ketone, preferably an aldehyde. The carbonyl compounds
according to the invention preferably have a low solubility in water of
less than 100 g/l, more preferably less than 50 g/l, particularly
preferably less than 30 g/l, in each case at 20° C. Alkyl and/or
aryl groups, which can also comprise further functional groups (for
example alcohol, ether, carbonyl, carboxylic acid groups, etc.) and can
be alky, alkylene or arylene groups interrupted by oxygen, sulfur or
nitrogen, are preferably bound to the carbonyl group of the carbonyl
compounds. Particular preference is given to aliphatic carbonyl compounds
which do not have any further heteroatoms in addition to the carbonyl
group. Suitable carbonyl compounds are, for example, pentanal,
2-methylpentanal, hexanal, 2-ethylhexanal, heptanal,
4-formyltetrahydropyran, 4-methoxybenzaldehyde, 4-tert-butylbenzaldehyde,
4-methylbenzaldehyde, glyoxal, glutaraldehyde, methylglyoxal,
cyclohexenone, cyclohexanone, diethyl ketone. Particularly preferred
carbonyl compounds are pentanal, 2-methylpentanal, hexanal and heptanal.

[0016] For the purposes of the present invention, an
α,β-unsaturated ester is an acrylic ester derivative which can
be substituted independently in positions 2 and 3, with two substituents
also being possible in position 3. The substituents are preferably alkyl
groups, halogen atoms, C1-C20-alkoxy groups, alkyl, alkylene or arylene
radicals interrupted by oxygen, sulfur or nitrogen, nitrile groups and
nitro groups. The substituents are preferably selected from the group
consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, tent-butyl,
trifluoromethyl, fluorine, chlorine, bromine, iodine, methoxy, ethoxy,
methylene, ethylene, propylene, isopropylene, benzylidene, nitrile and
nitro. Particular preference is given to substituents selected from the
group consisting of methyl, ethyl, methoxy, ethoxy. The
α,β-unsaturated ester is preferably a C1-C12-alkyl ester,
particularly preferably a C1-C5-alkyl ester, very particularly preferably
a methyl or ethyl ester. The α,β-unsaturated esters used
according to the invention preferably have a low solubility in water of
less than 100 g/l, preferably less than 50 g/l, particularly preferably
less than 20 g/l, in each case at 20° C.

[0017] α,β-Unsaturated esters and carbonyl compounds are the
starting materials for the reductive coupling according to the invention.

[0018] An aqueous electrolyte for the purposes of the present invention
comprises the starting materials together with water, at least one
electrolyte salt and at least one buffer as components. In addition, the
electrolyte preferably also comprises at least one complexing agent
and/or at least one anode corrosion inhibitor as further components. The
aqueous electrolyte in its totality with all components including the
starting materials will hereinafter also be referred to as reaction
electrolyte. The aqueous composition corresponding to the reaction
electrolyte without starting materials will hereinafter also be referred
to as supporting electrolyte. The aqueous reaction electrolyte has a
water content of preferably at least 20% by weight, particularly
preferably at least 50% by weight, in particular at least 75% by weight,
based on the total aqueous reaction electrolyte.

[0019] The reaction electrolyte according to the invention comprises at
least one electrolyte salt, selected from among bisquaternary and
multiquaternary ammonium and phosphonium salts, which suppresses the
cathodic formation of hydrogen. Preferably, apart from these
bisquaternary and multiquaternary ammonium and phosphonium salts, no
further electrolyte salts are used. In general, the electrolyte salt is
used in an amount in the range from 0.01 to 2.5% by weight, preferably
from 0.01 to 1.5% by weight, preferably from 0.01 to 0.5% by weight,
particularly preferably from 0.05 to 0.25% by weight, based on the total
aqueous reaction electrolyte. Particularly suitable electrolyte salts are
bisquaternary ammonium and phosphonium salts (EP 635587 A). Particular
preference is given to using bis(dibutylethyl)hexamethylenediammonium
hydroxide as electrolyte salt for the electrolyte. Possible counterions
are, for example, sulfate, hydrogensulfate, alkylsulfates, arylsulfates,
alkylsulfonates, arylsulfonates, halides, phosphates, carbonates,
alkyiphosphates, alkylcarbonates, nitrate, alkoxides, hydroxide,
tetrafluoroborate or perchlorate. The acids derived from the
abovementioned anions are also possible as electrolyte salts, i.e. for
example sulfuric acid, sulfonic acids and carboxylic acids. Ionic liquids
are also suitable as electrolyte salts. Suitable ionic liquids are
described in "Ionic Liquids in Synthesis", edited by Peter Wasserscheid,
Tom Welton, Verlag Wiley VCH, 2003, chapters 1 to 3, and also in DE
102004011427 A.

[0020] The reaction electrolyte further comprises at least one buffer
having a buffering range at a pH of from 7 to 11, preferably from 8 to
10, for buffering the protons formed in the anodic formation of oxygen.
Suitable buffers are, for example, hydrogenphosphate or
hydrogencarbonate, preferably in the form of their sodium salts.
Particular preference is given to using disodium hydrogenphosphate as
buffer for the electrolyte. In general, the buffer is used in an amount
in the range from 0.9 to 8% by weight, preferably from 4 to 7% by weight,
based on the total aqueous reaction electrolyte.

[0021] Furthermore, the reaction electrolyte preferably comprises one or
more anode corrosion inhibitors such as the borates known for this
purpose, preferably disodium diborate and orthoboric acid, in an amount
of from 0.4 to 3% by weight, preferably from 1 to 2% by weight, based on
the total aqueous reaction electrolyte.

[0022] Furthermore, the reaction electrolyte preferably comprises one or
more complexing agents in order to prevent the precipitation of iron and
lead ions. Mention may be made by way of example of
ethylenediaminetetraacetate (EDTA), triethanolamine (TEA), triethylamine,
nitrilotriacetate, preferably EDTA in an amount in the range from 0 to 1%
by weight, preferably from 0.1 to 0.5% by weight, based on the total
aqueous reaction electrolyte, and/or TEA in an amount in the range from 0
to 0.5% by weight, preferably from 0.05 to 0.2% by weight, based on the
total aqueous reaction electrolyte. Instead of TEA, it is possible to use
triethylamine in an amount of from 0 to 0.5% by weight, preferably from
0.05 to 0.2% by weight, based on the total aqueous reaction electrolyte.

[0023] As anode material, it is possible to use known anode materials; in
the case of undivided cells, materials having a low oxygen overvoltage,
for example carbon steel, glassy carbon, steel, mercury, cadmium,
platinum, iron, nickel, magnetite, lead, lead alloys or lead dioxide, are
usually used. Preference is given to using an anode composed of steel,
iron, lead or a lead alloy.

[0024] As cathodes, use is made of lead, lead alloys, cadmium, cadmium
alloys, mercury, steel, glassy carbon or boron-doped diamond electrodes.
Preference is given to using lead, lead alloys, cadmium, steel and glassy
carbon as cathode materials. Particular preference is given to using lead
and lead alloys as cathode materials.

[0025] In the aqueous reaction electrolyte according to the invention, the
organic starting materials (α,β-unsaturated esters and
carbonyl compounds) and the products formed (γ-hydroxycarboxylic
esters and γ-lactones) are present as organic phase of an emulsion.
The emulsion is maintained during the electrolysis by mechanical
agitation such as stirring or pump circulation of the electrolyte in the
electrolysis cell, or else by addition of suitable emulsifiers which
stabilize the emulsion. The emulsion is preferably maintained during the
electrolysis by mechanical agitation such as stirring or pump circulation
of the electrolyte. After the electrolysis, demixing of the emulsion can
be achieved, for example by stopping the agitation or by addition of a
suitable flocculent. After demixing of the emulsion to get an aqueous
phase and an organic phase, the products and any unreacted starting
materials can easily be separated off with the organic phase from the
aqueous electrolyte. This simplifies the separation of the products from
the electrolyte.

[0026] In the electrolysis of the invention, the starting materials
α,β-unsaturated esters and carbonyl compounds are preferably
used in an essentially equimolar ratio. The molar ratio of
α,β-unsaturated ester used to carbonyl compound used is
usually in the range from 0.25 to 4, preferably from 0.5 to 2,
particularly preferably from 0.8 to 1.2. While an excess of carbonyl
compound is used in the previously known processes for reductive coupling
of α,β-unsaturated esters with carbonyl compounds in order to
suppress the homo-coupling of the ester, the process of the invention
displays a high selectivity to the cross-coupling product of
α,β-unsaturated ester and carbonyl compound. When the starting
materials are used in an essentially equimolar ratio, particularly good
yields of the cross-coupling product can be achieved by means of the
process of the invention. The α,β-unsaturated ester is
preferably used in an amount of from 1 to 25% by weight, particularly
preferably from 5 to 10% by weight, based on the total aqueous reaction
electrolyte.

[0027] The electrolysis is usually carried out at a current density of at
least 1 A/dm2, preferably from 1 to 4 A/dm2. However, it is
also possible to carry out the electrolysis at a higher current density
of up to 20 A/dm2.

[0028] The electrolysis of the invention is usually carried out at a
temperature of from 20 to 60° C. and under atmospheric pressure.

[0029] The electrolysis can be carried out either continuously or
batchwise and in all conventional undivided electrolysis cells, for
example in glass beaker cells or plate cells and frame cells or cells
having fixed-bed or moving-bed electrodes. Both monopolar and bipolar
connection of the electrodes can be employed. The electrolyte in the
electrolysis cell is preferably circulated by pumping or stirred, as a
result of which its presence as emulsion can be maintained. Very
particularly suitable cells are capillary cells or plate stack cells
connected in a bipolar manner, in which the electrodes are configured as
plates and are arranged parallel to one another (Ullmann's Encyclopedia
of Industrial Chemistry, 2009 electronic release, VCH-Verlag Weinheim,
Volume Electrochemistry, Chapter 3, Electrochemical Cells and Chapter 5,
Organic Electrochemistry, Subchapter 5.4.3. Electrochemical Cells).

[0030] In an undivided electrolysis cell, anode space and cathode space
are not separated from one another by a membrane. Such undivided cells
are cheaper and technically easier to release. Particularly in the case
of organic processes, the use of divided cells can lead to rapid aging of
the membrane, which results in unsatisfactory operating lives.

[0031] In the process of the invention for the electrochemical reductive
cross coupling of α,β-unsaturated esters with carbonyl
compounds, the γ-lactone or the corresponding
γ-hydroxycarboxylic ester can in each case be formed either alone
or as a mixture. If necessary, any γ-hydroxycarboxylic ester formed
can be converted into the γ-lactone by transesterification after
the electrochemical reductive cross-coupling. The transesterification to
form the γ-lactone can, for example, be carried out by heating the
γ-hydroxycarboxylic ester in the presence of acid. If necessary,
the alcohol liberated can be removed from the reaction mixture in order
to shift the reaction in the direction of the γ-lactone.
Conversely, any γ-lactone formed can be converted into the
γ-hydroxycarboxylic ester by transesterification (alcoholysis), for
example by heating the γ-lactone in alkaline, nonaqueous alcoholic
solutions, after the electrochemical reductive cross-coupling. The
γ-hydroxycarboxylic ester can subsequently be converted further
into the free acid or the carboxylic acid salt by hydrolysis. For this
purpose, the γ-hydroxycarboxylic ester is, for example, heated with
aqueous alkaline solutions. As an alternative, the free
γ-hydroxycarboxylic acid or its salt can also be prepared directly
from the γ-lactone by hydrolysis. This can be carried out, for
example, by heating the γ-lactone in aqueous, alkaline solutions.

[0032] The invention further provides the γ-butyrolactone
derivatives of the general formula I

##STR00004##

where

[0033] R1, R2 and R3 are each, independently of one another, a hydrogen or
an alkyl group having from 1 to 5 carbon atoms, preferably a hydrogen, a
methyl or ethyl group, and R4 and R5 are alkyl groups having from 1 to 4
carbon atoms, preferably from 1 to 3 carbon atoms, with R4 and R5 being
identical radicals,

[0034] which can be prepared by the process of the invention.

[0035] The compounds of the formula I can be prepared by the
electrochemical cross-coupling according to the invention of
α,β-unsaturated esters of the formula II

##STR00005##

[0036] with 2-alkylalkanals of the formula III

##STR00006##

[0037] where R1 to R5 have the same meanings as in the compounds of the
formula I and R is an alkyl group, usually an alkyl group having from 1
to 12 carbon atoms, preferably from 1 to 5 carbon atoms, very
particularly preferably a methyl or ethyl group.

[0038] The invention preferably provides the y-butyrolactone derivatives
of the general formula IV

##STR00007##

where

[0039] R2 is a hydrogen or an alkyl group having from 1 to 5 carbon atoms,
preferably a hydrogen, a methyl group or an ethyl group, and R4 and R5
are alkyl groups having from 1 to 4 carbon atoms, preferably from 1 to 3
carbon atoms, with R4 and R5 being identical radicals, which can be
prepared by the process of the invention.

[0040] The compounds of the formula IV can be prepared by the
electrochemical cross-coupling according to the invention of
α,β-unsaturated esters of the formula II (where R1 and R3 are
in each case hydrogen) with 2-alkylalkanals of the formula III.

[0041] The 2-alkylalkanals of the formula III can be prepared, for
example, by aldol condensation of alkanals having from 3 to 6 carbon
atoms (propanal, butanal, pentanal or hexanal).

[0042] Particular preference is given to the γ-butvrolactone
derivatives 4-(2-pentyl)butyrolactone

##STR00008##

[0043] and 3-methyl-4-(2-pentyl)butyrolactone

##STR00009##

[0044] which can be prepared by the electrochemical cross-coupling
according to the invention of acrylic esters or crotonic esters with
2-methylpentanal.

[0045] The invention further provides the y-hydroxycarboxylic acids and
y-hydroxycarboxylic esters of the general formula VIII

##STR00010##

where

[0046] R1, R2, R3 and R7 are each, independently of one another, a
hydrogen or an alkyl group having from 1 to 5 carbon atoms, preferably a
hydrogen, a methyl group or an ethyl group, R is a hydrogen or an alkyl
group, usually a hydrogen or an alkyl group having from 1 to 5 carbon
atoms, and R8 is a branched alkyl group having from 3 to 10 carbon atoms,
which can be prepared by the process of the invention.

[0047] The invention preferably provides the y-hydroxycarboxylic acids and
y-hydroxycarboxylic esters of the general formula VIII, where R1, R2 and
R3 are each, independently of one another, a hydrogen or an alkyl group
having from 1 to 5 carbon atoms, preferably a hydrogen, a methyl group or
an ethyl group, R is a hydrogen or an alkyl group, usually a hydrogen or
an alkyl group having from 1 to 5 carbon atoms, R7 is a hydrogen and R8
is a branched alkyl group having from 3 to 10 carbon atoms, which can be
prepared by the process of the invention.

[0048] The compounds of the formula VIII can be prepared by the
electrochemical cross-coupling according to the invention of
α,β-unsaturated esters of the formula II with the carbonyl
compound of the formula VIII

##STR00011##

[0049] where R7 and R8 have the same meanings as in the compounds of the
formula VIII.

[0050] Alkyl groups for the purposes of the invention can in principle be
either branched or unbranched, either linear or cyclic and either
saturated or unsaturated (including multiply unsaturated). They
preferably have from 1 to 20, particularly preferably from 1 to 6, carbon
atoms. They preferably do not have any heteroatoms.

[0051] Aryl groups for the purposes of the invention are aromatic radicals
having preferably from 5 to 20 carbon atoms.

[0052] The invention further provides for the use of the
γ-butyrolactone derivatives of the formula I according to the
invention, preferably the γ-butyrolactone derivatives of the
formula IV, particularly preferably 4-(2-pentyl)butyrolactone or
3-methyl-4-(2-pentyl)butyrolactone as fragrances or flavors.
4-(2-Pentyl)butyrolactone has a pear-like aroma and
3-methyl-4-(2-pentyl)butyrolactone has a wood-like aroma.

EXAMPLES

[0053] The invention will now be illustrated by the following, nonlimiting
examples.

Example 1

[0054] Electrochemical preparation of ethyl y-hydroxypelargonate and
4-pentylbutyrolactone by reductive cross-coupling of ethyl acrylate with
hexanal using an excess of hexanal

[0055] Ethyl acrylate (1.7% by weight) and hexanal (29.7% by weight) were
emulsified in an aqueous electrolyte (0.16% by weight of
bis(dibutylethyl)hexamethylenediammonium hydroxide (bisquat), 0.38% by
weight of EDTA, 0.14% by weight of TEA, 1.45% by weight of Na2B4O7 and
5.84% by weight of Na2HPatin water at a pH of 10) (all % by weight
are based on the total aqueous reaction electrolyte) and subjected to
galvanostatic electrolysis at a current density of 2.23 A/dm2 and a
temperature of 20° C. in a pot cell. The current throughput was 2
F/mol of ester. A steel anode and a lead cathode were used as electrodes
(electrode area of 0.1 dm2 and spacing of 1 cm). To monitor the
reaction, the methyltributylammonium methylsulfate extract (MTBE extract)
of a sample of the electrolysis output was analyzed by gas
chromatography. A yield of 0.1% of 4-pentylbutyrolactone and a yield of
2.9% of the corresponding ethyl γ-hydroxypelargonate were achieved.
This corresponded to a total yield of target products 3.0% of the
theoretical yield.

Example 2

[0056] Electrochemical preparation of ethyl γ-hydroxypelargonate and
4-pentylbutyrolactone by reductive cross-coupling of ethyl acrylate with
hexanal using the starting materials in an equimolar ratio

[0057] Ethyl acrylate (5.9% by weight) and hexanal (6.0% by weight) were
reacted (all % by weight are based on the total aqueous reaction
electrolyte) and analyzed as described in example 1. A yield of
4-pentylbutyrolactone of 23.7% and a yield of the corresponding ethyl
γ-hydroxypelargonate of 48.0% were achieved. This corresponded to a
total yield of target products of 71.7% of the theoretical yield.

Comparative Example 1

[0058] Electrochemical preparation of ethyl y-hydroxypelargonate and
4-pentylbutyrolactone by reductive cross-coupling of ethyl acrylate with
hexanal using an excess of hexanal

[0059] Corresponding to the reductive coupling described by Burger, ethyl
acrylate (1.7% by weight) and hexanal (29.7% by weight) were dissolved in
an electrolyte (17.0% by weight of tetrabutylamine tetrafluoroborate
(Buar\lBF4) in a 3:1 mixture of dioxane and ethanol)(all % by weight are
based on the total aqueous reaction electrolyte) and subjected to
galvanostatic electrolysis at a current density of initially 2.23
A/dm2 and a temperature of 21° C. in a pot electrolysis cell.
The current throughput was 2 F/mol of ester. During the course of the
electrolysis, the current density dropped to 0.73 A/dm2. A platinum
anode and a graphite cathode were used as electrodes (electrode area of
0.1 dm2 and spacing of 1 cm). To monitor the reaction, the
methyltributylammonium methylsulfate extract of a sample of the
electrolysis output was analyzed by gas chromatography. A yield of
4-pentylbutyrolactone of 2.0% and a yield of the corresponding ethyl
y-hydroxypelargonate of 0.2% were achieved. This corresponded to a total
yield of target product of 2.2% of the theoretical yield.

Comparative Example 2

[0060] Electrochemical preparation of ethyl y-hydroxypelargonate and
4-pentylbutyrolactone by reductive cross-coupling of ethyl acrylate with
hexanal using the starting materials in an equimolar ratio

[0061] Ethyl acrylate (5.9% by weight) and hexanal (6.0% by weight) were
reacted at 22° C. (all % by weight are based on the total aqueous
reaction electrolyte) and subsequently analyzed as described in
comparative example 1, with the current density remaining constant during
the experiment. A yield of 4-pentylbutyrolactone of 16.5% and a yield of
the corresponding ethyl γ-hydroxypelargonate of 0.7% were achieved.
This corresponded to a total yield of target products of 17.2% of the
theoretical yield.

Example 3

[0062] Preparation of Whiskey Lactone

[0063] Ethyl crotonate (6.9% by weight) and pentanal (5.2% by weight) were
emulsified in an aqueous electrolyte (0.16% by weight of
bis(dibutylethyl)hexamethylenediammonium hydroxide (bisquat), 0.38% by
weight of EDTA, 0.14% by weight of TEA, 1.45% by weight of Na2B4O7 and
5.84% by weight of Na2HPO4 in water at a pH of 10) (all % by weight based
on the total aqueous reaction electrolyte) and subjected to galvanostatic
electrolysis at a current density of 2.23 A/dm2 and a temperature of
25° C. in a frame electrolysis cell. The current throughput was 2
F/mol of ester. A steel anode and a lead cathode were used as electrodes
(electrode area of 0.1 dm2 and spacing of 1 cm). To monitor the
reaction, the MTBE extract of a sample of the electrolysis output is
analyzed by gas chromatography. A yield of 3-methyl-4-butylbutyrolactone
(whiskey lactone) of 68.5% and a yield of the corresponding ethyl
y-hydroxycarboxylate of 24.2% were achieved. This corresponded to a total
yield of target products of 92.7% of the theoretical yield.

Example 4

[0064] Preparation of 4-(2-pentyl)butyrolactone

[0065] Ethyl acrylate (5.9% by weight) and 2-methylpentanal (6.0% by
weight) were reacted electrochemically using a method analogous to
example 3 (all % by weight are based on the total aqueous reaction
electrolyte). A yield of 4-(2-pentyl)butyrolactone of 88.4% was achieved.

Example 5

[0066] Preparation of 3-methyl-4-(2-pentyl)butyrolactone

[0067] Ethyl crotonate (6.7% by weight) and 2-methylpentanal (5.9% by
weight) were reacted electrochemically by a method analogous to example 3
(all % by weight are based on the total aqueous reaction electrolyte). A
yield of 4-(2-pentyl)butyrolactone of 66.7% and a yield of the
corresponding ethyl y-hydroxycarboxylate of 26.6% were achieved. This
corresponded to a total yield of target products of 93.2% of the
theoretical yield.

Example 6

[0068] Preparation of 3,3-dimethyl-4-pentylbutyrolactone

[0069] Methyl 3,3-dimethylacrylate (4.9% by weight) and hexanal (4.3% by
weight) were reacted electrochemically by a method analogous to example 3
(all % by weight are based on the total aqueous reaction electrolyte). A
yield of 3,3-dimethyl-4-pentylbutyrolactone of 37.2% was achieved.

Example 7

[0070] Preparation of 2-methyl-4-butylbutyrolactone

[0071] Ethyl methacrylate (6.7% by weight) and pentanal (5.2% by weight)
were reacted electrochemically using a method analogous to example 3 (all
% by weight are based on the total aqueous reaction electrolyte). A yield
of 2-methyl-4-butylbutyrolactone of 81.0% was achieved.

Example 8

[0072] Preparation of 2-methyl-4-(2-pentyl)butyrolactone

[0073] Ethyl methacrylate (6.7% by weight) and methylpentanal (5.9% by
weight) were reacted electrochemically using a method analogous to
example 3 (all % by weight are based on the total aqueous reaction
electrolyte). A yield of 2-methyl-4-(2-pentyl)butyrolactone of 70.9% was
achieved.

Patent applications by Florian Stecker, Mannheim DE

Patent applications by Itamar Michael Malkowsky, Speyer DE

Patent applications by Olivier Abillard, Mannheim DE

Patent applications by Ralf Pelzer, Fuerstenberg DE

Patent applications by Simone Lutter, Ludwigshafen DE

Patent applications by BASF SE

Patent applications in class The lactone ring is five-membered

Patent applications in all subclasses The lactone ring is five-membered